An adjusting mechanism of the above-mentioned type is illustrated for example in Austrian Patent No. 330 633 (which corresponds to U.S. Pat. No. 4,022,493). In this known design, a locking part which extends through recesses in the guide rail carries at least one serrated strip which can be engaged or disengaged from outside with a serrated strip provided on a stepping plate which is coupled with the base plate of the jaw. Also, it is possible in the case of such a jaw to support the locking part on the base plate, whereby a serrated strip is then provided on the guide rail. An important disadvantage of this adjusting mechanism is that structural changes to the guide rail or the base plate are needed, which require reinforcement of these structural parts in the area of the locking piece, thereby causing the entire adjusting mechanism to be expensive to build. Even though the spring which biasses the locking part can be selected to be relatively weak, as compared to springs of other known adjusting mechanisms, it nevertheless must, since the locking part projects laterally outwardly beyond the guide rail and the jaw in order to facilitate operation of the adjusting mechanism, be sufficiently strong that inadvertent disengagement during handling of the jaw cannot occur. Thus, an easy operation of the adjusting mechanism, either with an operating tool or by hand, is not possible.
Therefore, a basic purpose of the invention is to provide an adjusting mechanism of the above-mentioned type which requires minimal structural changes of the jaw and in which the unlocking of the jaw position can be done against the force of a very weak spring.